Four hurricanes hit Florida in 2004, dousing the state with some of its heaviest rainfall in more than three decades. All of that water, a team of researchers says, may have set the stage for a large “red tide” off the state’s west-central coast in 2005 by flushing an excessive amount of nutrients into the Gulf of Mexico through both rivers and undersea groundwater.

Nearly annual events off Florida’s Gulf coast, red tides are massive algal blooms caused by Karenia brevis, which thrives on nutrients such as nitrogen and phosphorous. Typically, Florida’s red tides occur in small patches that last “at most a few months, and then they go away,” says Frank Muller-Karger, a biological oceanographer at the University of South Florida, in St. Petersburg.

But the red tide of 2005 lasted a year and, at times, covered 67,500 square kilometers, making it “one of the longest red tide events in history, and perhaps the most extensive one, in terms of size, in history,” says Chuanmin Hu, a satellite oceanographer also at the University of South Florida.

In the past, the scientific community has often focused on surface water runoff — which picks up nutrients as it makes its way to the sea over the course of several months — as a source of nitrogen and phosphorous for red tides. But in a study published in the June 1 Geophysical Research Letters, Hu and Muller-Karger, along with Peter Swarzenski, a chemical oceanographer at the U.S. Geological Survey in St. Petersburg, Fla., say that even the elevated river runoff resulting from the 2004 hurricanes would not have provided enough nutrients to sustain a red tide the size and duration of last year’s.

The researchers suggest that an additional source of nutrients could be Florida’s many undersea springs and “seeps” — openings in the ocean floor where groundwater flows into the sea; the groundwater picks up nutrients as it moves through rock. According to the team’s research, the amount of nitrogen that seeps contribute to central and south Florida’s coastal waters in the Tampa Bay area alone is about one-third of the contribution from all the rivers draining to Florida’s Gulf coast combined.

Muller-Karger says Florida’s heavy rainfall in 2004 likely increased pressure on stores of groundwater, causing more groundwater — and therefore more nutrients — to flow through the seeps. And because groundwater flows to the sea more slowly than surface water, he adds, nutrients could have been delivered over an extended period, thereby allowing the red tide to persist through the entire next year.

“They’re on to an important point,” says Mandy Joye, a biogeochemist at the University of Georgia in Athens. “It’s not a stretch at all to imagine that groundwater-derived nutrients are fueling these blooms.” Joye says scientists have tended to focus on surface water because groundwater is an “out-of-sight, out-of-mind issue,” but that the team’s study will “raise some eyebrows” and hopefully draw attention to groundwater.

Muller-Karger, too, says scientists need to give greater consideration to springs and seeps as potential nutrient sources for red tides. “We need to look at all the nutrient sources,” he says, because until scientists better understand what fuels the algal blooms, “there’s nothing we can do to mitigate them.”

Red tides are problematic because K. brevis produces neurotoxins that kill marine creatures and are an eye and respiratory irritant in humans. The blooms also impact the state’s tourist industry: Last year’s red tide cost the Florida Gulf Coast millions of dollars in revenue.

So far this year, Muller-Karger says, “the red tides are very small” — a seeming contradiction given last year’s intense hurricane season. However, he explains, last year’s hurricanes mostly affected the northern Gulf area rather than central Florida.